1. Field of the Invention
The present invention relates to an acceleration sensor, and more particularly to an acceleration sensor for detecting an acceleration caused by an object with a piezoelectric element mounted on an oscillation plate accommodated in a sensor casing.
2. Description of the Related Art
In general, the acceleration sensor now known and in use includes various types such as an electromagnetic type, a piezoelectric element type, and a semiconductor type, all of which are designed to detect the acceleration. Among these types of acceleration sensors, the piezoelectric element type of acceleration sensor is known as detecting acceleration with a piezoelectric element when it is deformed to generate a voltage indicative of the acceleration. These types of acceleration sensors are usually mounted on automobiles to be used for controlling knockings of engines and airbag systems.
The acceleration sensor of this type is raised for example as a first conventional acceleration sensor and shown in FIGS. 25 and 26. The acceleration sensor 800 comprises a fixed case member 801, an oscillation plate 802, a piezoelectric element 803, electrodes 804, a metal wire 805, a cover member 806, an output terminal pin 807 and a resilient ring 808. The fixed case member 801 formed in a cylindrical shape is made of a metal and has a supporting portion 801a upwardly projecting from and integrally formed with the bottom portion of the fixed case member 801. The oscillation plate 802 formed in an annular shape is made of a metal and securely mounted on the supporting portion 801a of the fixed case member 801 by welding. The piezoelectric element 803 formed in an annular shape is provided on the oscillation plate 802 in axial alignment with the oscillation plate 802. The piezoelectric element 803 is covered with the electrodes 804. One of the electrodes 804 is electrically connected with the oscillation plate 802, while the other of the electrodes 804 is electrically connected with the output terminal pin 807. The electrical connection between the other of the electrodes 804 and the output terminal pin 807 is established by the metal wire 805 having both ends soldered at 805a by wire bonding and like. The cover member 806 formed in a cylindrical shape is made of a plastic material and has an exterior object mounted thereon and electrically connected with the output terminal pin 807. The output terminal pin 807 is mounted on the cover member 806. The fixed case member 801 and the cover member 806 have respectively peripheral edge portions 801c and 806c bent and fixedly coupled with each other with the resilient ring 808 intervening between the peripheral edge portions 801c and 806c to hermetically seal the gap and define a closed space in which the oscillation plate 802 and the piezoelectric element 803 are operatively accommodated. Therefore, no water enters the closed space through the gap.
Another acceleration sensor of the piezoelectric element type is raised for example as a second conventional acceleration sensor and shown in FIG. 27.
The acceleration sensor 900 comprises a fixed case member 901, a metal base member 902, an oscillation plate 802, a piezoelectric element 803, electrodes 804, a metal plate 903, a cover member 904, an output terminal pin 807 and a resilient ring 905. The fixed case member 901 formed in a cylindrical shape has an annular ledge portion 901c radially inwardly bent. The metal base member 902 formed in a circular shape and provided on the fixed case member 901 at the annular ledge portion 901c of the fixed case member 901. The cover member 904 formed in a circular shape has a peripheral edge portion 904a fixedly connected with the annular ledge portion 901c of the fixed case member 901 with the metal base member 902 intervening between the fixed case member 901 and the cover member 904. The fixed case member 901, the metal base member 902 and the cover member 904 collectively define a closed space to accommodate the oscillation plate 802 and the piezoelectric element 803 to be oscillatable by an oscillation exerted on the acceleration sensor. On the cover member 904 formed in a circular shape is mounted the output terminal pin 807 electrically connected with the piezoelectric element 803 and connectable with an exterior connecting member. The metal base member 902 has a supporting portion 902a projecting toward the fixed case member 901 into the closed space and has the oscillation plate 802 and the piezoelectric element 803 securely supported thereon. In this example, both of the oscillation plate 802 and the piezoelectric element 803 are formed in an annular shape, and the cover member 904 is made of a plastic material to ensure that the output terminal pin 807 is electrically insulated from the metal base member 902. Through the supporting portion 902a of the metal base member 902 is extending the output terminal pin 807 which has one end electrically connected with one of the electrodes 804 of the piezoelectric element 803 through the metal plate 903 soldered by 903a and thus electrically connected with one of the electrodes 804 of the piezoelectric element 803 so that the oscillation plate 802 and the piezoelectric element 803 can be oscillated when they are exerted by an acceleration. The resilient ring 905 is interposed between the inner surface of the fixed case member 901 and the outer surface of the metal base member 902 to ensure that the resilient ring 905 hermetically seals the closed space. The rigidity of the metal plate 903 is preferably as small as possible and may be replaced by the metal wire 805 electrically connected with the electrode 804 of the piezoelectric element 803 and the output terminal pin 807, while the oscillation plate 802 may be connected to the supporting portion 902a by welding.
The above two type of acceleration sensors 800 and 900 have male screws 801b and 901b, respectively formed on its exterior side of the fixed case member 801 and 901 to be screwed into a female screw portion formed in a detectable object such as engine. Thus, the oscillation plate 802 is oscillated and deformed by an oscillation from the detectable object such as engine to have the piezoelectric element 803 generate a voltage indicative of the acceleration, thereby enabling the voltage to be outputted from the electrodes 804 through the output terminal pin 807 with the fixed case member 801, 901 and the metal base member 902 earthed to the ground.
FIG. 28 is a graph showing a characteristic of the resonance frequency fo with respect to the oscillation under a predetermined acceleration of the acceleration sensor of these types, for example, obtaining a relatively high sharpness of resonance Q in the vicinity of a point of the resonance frequency fo while obtaining a relatively low and flat sharpness of resonance Q at intermediate and lower frequency range. Here, the sharpness of resonance Q means sensitivity of resonance. Generally available is the relatively high sharpness of resonance Q at around the point of the resonance frequency fo and the relatively low and flat sharpness of resonance Q at intermediate and lower frequency range any one of which is selected depending upon the acceleration sensor in use. Accordingly, the upper limit of the frequency range in substantial use is the point of the resonance frequency fo. For example, the sharpness of resonance Q in the vicinity of the point of the resonance frequency fo used for obtaining the desirable frequency makes it impossible to detect a frequency slightly out of the point of the resonance frequency fo. Generally, the disadvantages inherent in the foregoing apparatus is overcome with the resistance R and the piezoelectric element 803 connected in parallel relationship with each other to have the output voltage kept at relatively low level as shown in FIG. 29, thereby reducing the sharpness of the resonance Q to an appropriate value as indicated in a broken line in FIG. 28. In aspect of the sensitivity, the acceleration sensor 900 shown in FIG. 27 is found to be of a higher sensitivity than that of the acceleration sensor 800 shown in FIG. 25 through repeated experiments. This reason is considered to be due to the fact that the oscillation plate 802 is supported by the metal base member 902 so that the metal base member 902 without a perfect rigidity is oscillated together with the oscillation plate 802 by the acceleration exerted on the oscillation plate 802 and the metal base member 902, thereby making it possible the oscillation plate 802 to serve as an amplifying transformer. This type of the acceleration sensor is disclosed in the Japanese Patent Laid-Open Publication No. S58-142227.
The electrodes of the piezoelectric element 803 may include two different types such as a stimulus electrode with small diameter and a stimulus electrode with large diameter, which are positioned in coaxial alignment with an oscillation direction to receive the acceleration. An alternating current voltage from an exterior object is transmitted through the stimulus electrodes to deform the piezoelectric element 803, which enables the oscillation plate 802 to be oscillated. The oscillation of the oscillation plate 802 produces an electric potential from the electrodes 804 so that the function and failure of the acceleration sensor, and levels of the detection can be checked.
The previously mentioned conventional acceleration sensors 800 and 900 are of the type that the oscillation plate 802 is supported by the supporting portions 801a or 902a. Besides this type of the acceleration sensor, there are various types of acceleration sensor, for example, the type the oscillation plate is in the form of a circular shape and has a peripheral edge portion clamped and the type the oscillation plate is in the form of a rod shape and has one end fixed and the other end freely oscillatable in a cantilever fashion. Further, the above conventional acceleration sensors comprise, for example, the type between the electrodes 804 of the piezoelectric element 803 and the output terminal pin 807 is provided a print base plate accommodating therein an electric impedance transformer, an amplifier, a correction circuit and other electronic parts all of which are electrically connected with the metal wire 805. The above conventional acceleration sensors still further comprise the type having a single output terminal pin 807 provided in association with the fixed case member 801 and 901 to serve as an earth member. The other type of acceleration sensor having double terminal pins is known.
However, the acceleration sensors of the prior art possess their own distinct limitations. Generally, as shown in FIG. 30, the oscillation plate 802 and the piezoelectric element 803 of those acceleration sensors have resonance characteristics in the vicinity of the point of the resonance frequency fo. However, in the case of those conventional acceleration sensors, an acoustic standing wave can be generated in a certain size of the closed space in which the oscillation plate 802 and the piezoelectric element 803 are oscillatably accommodated. As shown in FIG. 31, in the event of generating two peaks of resonance in the vicinity of the point of the resonance frequency fo, a large anti-resonance peak (hereinafter xe2x80x9cdipxe2x80x9d) can be generated because of their phase difference. This large dip can be the cause of spurious noise which deteriorates the characteristic of an acceleration sensor. In addition, in this case of those conventional acceleration sensors, an acoustic resonance can be generated in the closed space, which can be the cause of generating a dip. This dip can be also the cause of spurious noise which deteriorates the characteristic of an acceleration sensor.
A As this spurious noise is generated by sound, the frequency of generating spurious noise varies according to the sonic speed u. For example, the sonic speed increases 1.18 times when the temperatures change from 20 to 120, which can be derived from the following equation.
From this equation, it is understood that a large dip that cannot be generated in room temperatures can sometimes be generated in high temperatures. On the contrary, a large dip that was small in high temperatures can also sometimes be generated in room temperatures. As the reason for generating spurious noise has not been solved, the conventional acceleration sensor has to be designed to have the desirable resonance frequency fo. In addition, the constructing of a conventional acceleration sensor is a complicated process, that is, the acceleration sensor has to be customized to have a structure to avoid spurious noise, which needs repeated change of the dimensions of the acceleration sensor components.
The acceleration sensor has the resonance frequency fo in the usable frequency range or broad frequency range. The complicated process described above causes another problem, that is, it is extremely difficult to design the sensor casing of the acceleration sensor to have standardized dimensions.
An object of the present invention is to solve the above mentioned problems and to provide an acceleration sensor at low cost by the implementation of decreasing the influence of the anti-resonance, dip, with simple structure, wherein the acceleration sensor still keeps its high level of performance. Specifically, the acceleration sensor works most effectively when it is used around its resonance frequency, fo.
In accordance with a first aspect of the present invention, there is provided an acceleration sensor for detecting an acceleration caused by an object oscillated in an oscillation direction, comprising a sensor casing, an oscillation plate and a piezoelectric element. The sensor casing has a center axis and is positioned in coaxial alignment with the oscillation direction to receive the acceleration, the sensor casing has a first and second circular inner surfaces opposing to and spaced apart along the center axis from each other at a first space distance, and a third cylindrical inner surface connected at one end with the first inner surface and at the other end with the second inner surface to define a cylindrical closed space. The oscillation plate is accommodated in the closed space of the sensor casing and has a central portion securely supported by the sensor casing and a peripheral portion integrally formed with the central portion and extending radially outwardly of the central portion to be freely movable with respect to the sensor casing. The oscillation plate has a peripheral end surface spaced apart from the third inner surface of the sensor casing at an annular gap small enough to enable the oscillation plate to oscillate with respect to the sensor casing. The oscillation plate also has a first flat surface opposing to and spaced apart along the center axis from the first inner surface of the sensor casing at a second space distance, and a second flat surface opposing to and spaced apart along the center axis from the second inner surface of the sensor casing at a third space distance, with the oscillation plate being partly oscillatable along the center axis with respect to the sensor casing. The piezoelectric element has a first and second surface and is provided on at least one of the first and second flat surfaces of the oscillation plate to generate a voltage indicative of the acceleration. The first space distance is less than or equal to the diameter of the third inner surface of the sensor casing multiplied by 0.1.
In accordance with a second aspect of the present invention, there is provided an acceleration sensor for detecting an acceleration caused by an object oscillated in an oscillation direction, comprising a sensor casing, an oscillation plate, a first piezoelectric element and a second piezoelectric element. The sensor casing and oscillation plate are the same as in the first aspect of the invention. The first piezoelectric element has first and second surfaces and is provided on the first flat surface of the oscillation plate to generate a voltage indicative of the acceleration, and the second piezoelectric element has first and second surfaces and is provided on the second flat surface of the oscillation plate to generate a voltage indicative of the acceleration. The first space distance is less than or equal to the diameter of the third inner surface of the sensor casing multiplied by 0.1.
In accordance with a third aspect of the present invention, there is provided an acceleration sensor for detecting an acceleration caused by an object oscillated in an oscillation direction, comprising a sensor casing, an oscillation plate, and a piezoelectric element. The sensor casing includes a cylindrical fixed case member having a circular bottom portion having a first circular inner surface, a cylindrical side portion integrally formed with the bottom portion, and a supporting portion projecting from the bottom portion, a cover member being provided on the fixed case member and having a circular cover portion having a second circular inner surface; and a cylindrical side portion integrally formed with the cover portion. The side portion of the fixed case member has a third cylindrical inner surface connected at one end with the first inner surface, and the side portion of the cover member has a fourth cylindrical inner surface connected at one end with the second inner surface, with the second inner surface of the cover portion of the cover member opposing to and spaced apart along the center axis from the first inner surface of the bottom portion of the fixed case member at a first space distance. The first inner surface of the bottom portion of the fixed case member, the third inner surface of the side portion of the fixed case member, the second inner surface of the cover portion of the cover member, and the fourth inner surface of the side portion of the cover member collectively define a cylindrical closed space. The oscillation plate is accommodated in the closed space of the sensor casing and has a central portion securely supported by the supporting portion of the fixed case member of the sensor casing, and a peripheral portion integrally formed with the central portion and extending radially outwardly of the central portion. The oscillation plate has a first flat surface opposing to and spaced apart along the center axis from the first inner surface of the bottom portion of the fixed case member at a second space distance, and a second flat surface opposing to and spaced apart along the center axis from the second inner surface of the cover portion of the cover member at a third space distance. The piezoelectric element has a first surface held in contact with the second flat surface of the oscillation plate, and a second surface opposing to and spaced apart along the center axis from the second inner surface of the cover portion of the cover member at a fourth space distance. The piezoelectric element is provided on the second flat surface of the oscillation plate in axial alignment with the oscillation plate to generate a voltage indicative of the acceleration. The first space distance is less than or equal to the diameter of the third inner surface of the side portion of the fixed case member multiplied by 0.1, and in which the first space distance is less than or equal to the diameter of the fourth inner surface of the side portion of the cover member multiplied by 0.1.
In accordance with a fourth aspect of the present invention, there is provided an acceleration sensor for detecting an acceleration caused by an object oscillated in an oscillation direction, comprising a sensor casing, an oscillation plate and a piezoelectric element. The sensor casing includes a cylindrical fixed case member, a metal base member, and a cover member. The cylindrical fixed case member has a circular bottom portion having a first circular inner surface, and a cylindrical side portion integrally formed with the bottom portion having a first section close to the bottom portion of the fixed case member, a second section remote from the bottom portion of the fixed case member and radially inwardly bent, and an annular ledge section formed between the first and second sections with an annular ledge. The metal base member has a circular base portion and a supporting portion with the base portion having a second circular inner surface and a circular outer surface, and the supporting portion projecting from the second inner surface. The base portion of the metal base member has a central section integrally formed with the supporting portion, and a peripheral section extending radially outwardly of the central section. The metal base member is mounted on the annular ledge of the fixed case member with a resilient ring intervening between the second section of the side portion of the fixed case member and the peripheral section of the base portion of the metal base member to hermetically seal the gap between the second section of the side portion of the fixed case member and the peripheral section of the base portion of the metal base member. The first section of the side portion of the fixed case member has a third cylindrical inner surface connected at one end with the first inner surface of the bottom portion of the fixed case member and at the other end with the second inner surface of the base portion of the metal base member, with the second inner surface of the base portion of the metal base member opposing to and spaced apart along the center axis from the first inner surface of the bottom portion of the fixed case member at a first space distance. The cover member is provided on the outer surface of the metal base member and has a peripheral section firmly engaged with the second section of the side portion of the fixed case member. The first inner surface of the bottom portion of the fixed case member, the second inner surface of the base portion of the metal base member, and the third inner surface of the first section of the side portion of the fixed case member collectively define a cylindrical closed space. The oscillation plate accommodated in the closed space of the sensor casing and having a central portion securely supported by the supporting portion of the metal base member of the sensor casing, and a peripheral portion integrally formed with the central portion and extending radially outwardly of the central portion. The oscillation plate has a first flat surface opposing to and spaced apart along the center axis from the first inner surface of the bottom portion of the fixed case member at a second space distance, and a second flat surface opposing to and spaced apart along the center axis from the second inner surface of the base portion of the metal base member at a third space distance. The piezoelectric element has a first surface opposing to and spaced apart along the center axis from the first inner surface of the bottom portion of the fixed case member at a fourth space distance, and a second surface held in contact with the first flat surface of the oscillation plate. The piezoelectric element being provided on the first flat surface of the oscillation plate in axial alignment with the oscillation plate to generate a voltage indicative of the acceleration. The first space distance is less than or equal to the diameter of the third inner surface of the first section of the side portion of the fixed case member multiplied by 0.1.